When should I stop irrigating?

Sep 13

This time of year, growers often ask, “When should I stop irrigating?” It is an important question because terminating irrigation too early could lead to lower crop yields, while over-irrigating is a waste of water and money.

So how should you determine when it is time to stop irrigating? Keep reading to find out!

Field Corn

Bottom line, irrigate until corn reaches maturity. This happens once the black layer forms at the base of the kernels. This line forms a few days after the milk line reaches the kernel base.

In many cases, we see growers cut the water off too early, causing the corn to mature faster. This means the kernels will never reach their full potential in terms of size and weight. At the dent stage, corn has reached 75% of its weight. Stopping irrigation at the dent stage can cause yields to suffer as much as 15-20%!

Termination of corn irrigation can be determined by having an understanding of its life cycle, watching the milk line, and checking the soil moisture in your field.

Various milk line stages. Credit: Pioneer Corn Growth & Development.

Generally, corn will mature 60-75 days after silking. Once the milk line appears and begins its move through kernel, there are only 21-24 days until maturity. Near the end of the season, corn will only require about 0.06 inches of water per day. If the milk line is 75% down the kernel, the weather is not too hot or dry, and sensors indicate good soil moisture, irrigation on pivot irrigated fields could be terminated. If the weather is hot and dry, however, it will likely benefit your crops to keep watering until the black layer is reached and no additional moisture is entering the kernels. Remember that at maturity, kernel moisture will be about 35%.

Sweet Corn

Like field corn, irrigation for sweet corn should terminate at maturity. For sweet corn, this could be anywhere from 60 to 100 days. Vague, right? We all know that the sweet, juicy flavor of ripe sweet corn does not last long. There is really only one maximum yield date for sweet corn, so it is not only important to harvest at the right time, but that irrigation termination also occurs at the right time. Knowledge of your variety can help, but there are other ways to maximize yield and minimize irrigation costs.

Sweet corn tastes the sweetest right before the natural sugars convert to starch. There are multiple informal ways to see when sweet corn is reaching maturity. Tasting it uncooked is one option, but we do not recommend that.

Inspect Husk / Kernel: You can pull the husk back and check, but beware that once that husk is pulled back, it leaves the husk exposed to pests. If you do this and see pests already, it is likely mature or very close. Like us, pests like that sweet, juicy taste of ripe sweet corn. Another way to tell if you pull an ear back is to poke a kernel with your fingernail. If it does not squirt out, it is likely to young. If it squirts out a thick, pasty texture, it is too mature. But if it squirts and hits you in the eye, its probably perfectly mature.
Check Appearance: Ripe sweet corn will have dark, green leaves covering them. Additionally, tassels for mature sweet corn begin to dry out and turn brown.
Check the Ear:The best non-scientific approach for checking sweet corn maturity is to feel the top of the ear. If the kernels are exposed because the ear is bulging out of its husk, it is likely too mature. If you feel near the top and don’t feel any solid matter, the ear is not mature or ripe. But if the ear is filled out, and not exposed, that is the perfect time to stop irrigating and harvest. Remember that often times one stalk of sweet corn can have essentially two harvests. The top ear will generally mature slower than the ear closer to the soil.

Cotton

It’s hard to determine when to schedule your final irrigation in cotton. If you continue to irrigate, there is an increased risk for boll rot and hardlock, boll maturity could slow, and defoliants may be less effective. Cut water off too soon and you risk yield loss.

Consider the number of open bolls, the location of harvestable bolls, the weather, and the soil type (and associated soil water holding capacity) when deciding to stop irrigating your cotton fields.

Ultimately, you should stop irrigating if:

The soil has enough water to last until defoliation, or
Roughly 10% of bolls have opened

Generally speaking, for furrow irrigated cotton, terminate irrigation when cotton reaches first cracked boll. Since central pivot systems deliver less water at any given time compared to furrow irrigation, you can wait a bit longer to terminate: around 10 days after the first cracked boll.

Keep in mind, once bolls start opening, cotton only needs 0.5” of water a week. Allowing the plants to be water stressed accelerates boll opening, makes defoliation easier, and reduces regrowth.

Peanuts

Peanuts require careful decisions about terminating irrigation close to harvest. In terms of when to terminate irrigation, it is important to consider weather conditions (natural rainfall), soil moisture, and the maturity level of your peanuts. Once peanuts reach about 120 days after planting (DAP), you should continually check the maturity and decrease the amount of irrigation quite significantly. At 120 DAP, peanuts require around 0.7 inches of water a week, but the amount of water required drops roughly 0.2 inches per week.

To assess maturity, use one of the following methods:

Maturity Board Method: Remove 2 or 3 plants from a representative part of your field. Remove all full-size pods (there should be 100 to 200 of them). Make sure you remove all pods, not just those that look the most mature. Scrape or blast the outer surface of the each pod and organize on the Peanut Profile Board according to the mesocarp color. The board uses the color to predict the number of days before you should start digging.
Shell Out Method: Twist or break open pods. When 65% of pods have darkening on the inside of the hull and you can see veins on the seed coat, the peanuts have reached maturity. This is commonly used on Virginia type peanuts.

Over-irrigating after maturity won’t necessarily damage the peanuts, but it can significantly hamper digging them. Along with monitoring irrigation in terms of maturity, the soil moisture can affect the success of digging the peanuts. Dry soil can cause an issue with maintaining the diggers necessary depth, while too much irrigation can delay being able to get back into the field to dig while peanuts are at optimal maturity.